This invention relates to magnetic flowmeters with empty tube detection circuitry.
Magnetic flowmeters measure the rate of flow of a process fluid through a tube. Magnetic coils mounted on opposite sides of the tube produce a magnetic field perpendicular to the direction of fluid flow in the tube. Electrodes placed in the tube measure a resulting current in the fluid that is perpendicular to both the direction of fluid flow and the magnetic field. A processor converts the output of the electrodes to a measure of the rate of fluid flow.
Some magnetic flowmeters are equipped with a circuit that detects the presence of fluid in the tube and prevents the flowmeter from measuring a non-zero flow rate when the tube is empty. One such circuit, manufactured by Fischer and Porter, produces an oscillating signal to indicate that the tube is full. The oscillating signal disappears when the tube is empty.